1. Field of the Invention
This invention relates to an FET (Field Effect Transistor) amplifier circuit capable of compensating for the change in phase and/or magnitude of a signal passing through the circuit and an FET amplifier circuit also capable of protecting the circuit against over-amplitude or over-voltage signals applied thereto.
2. Description of the Prior Art
Referring to FIG. 1, there is shown the general arrangement of an FET amplifier circuit in the prior art, wherein reference numeral 1a, 1b and 1c denote a unity amplifier using FET, and numerals 2 and 3 denote an input terminal and an output terminal, respectively.
As is known in the art, FET amplifiers which are in particular intended for amplifying microwave signals with large amplitude have been getting increasingly useful because of their advantages in comparison with traveling-wave tube amplifiers. Such advantages include, for example, the fact that distortion is made lower by virtue of low phase shift and unsaturation characteristics, the reliability is high by using solid-state devices, the size is small, and the weight is light. These advantages are effective particularly in a satellite communication system using microwaves.
However, the FET amplifiers of the prior art involve some disadvantages. That is, the power dissipation depends on the power-added efficiency of an amplifier which processes high-level microwave signals. It is necessary for the FET amplifier to operate in low power dissipation conditions, that is, in a highly efficient state. As a result, the linearity of the amplifier must be sacrificed. This sacrifice of linearity causes a deterioration in the distortion and phase-change characteristics.
FIG. 2 shows phase-change and output amplitude characteristics versus relative input power to the FET amplifier. It is seen that the relative output power gets saturated as the relative input power increases and the amount of phase-change remarkably increases as the linearity of the amplifier is lost.
FET amplifiers often include means for protecting the amplifiers against the effect of over-amplitude input signals. FIG. 3 is a schematic diagram showing an exemplary prior art FET amplifier having protection against the effect of over-amplitude signals. In FIG. 3, the amplifier includes an FET 21 having a gate and a drain electrodes connected to an input matching circuit 22a and an output matching circuit 22b, respectively. The gate and drain electrodes are provided with bias circuits 23a and 23b, respectively, which supply those electrodes with direct-current bias and prevent the applied signals from flowing to a gate bias terminal 24 and a drain bias terminal 25 through which the direct-current biases are applied. A PIN diode 26 is connected to the amplifier in parallel for the purpose of limiting the amplitude of signals applied to the input matching circuit 22a. The PIN diode 26 is provided with direct-current bias from a bias circuit 27 which functions in a manner similar to the bias circuits 23a and the direct-current bias is applied through a diode bias terminal 28. In addition, blocking capacitors 29a, 29b and 29c are connected so as to block the flow of bias current for the FET 21 and the PIN diode 26 towards other elements while allowing signal components to pass. The signals are supplied to an input terminal 30 and are amplified by the FET 21 and then supplied from an output terminal 31. If a signal source connected to the input terminal 30 is broken down or an erroneous operation is performed, undesired over-amplitude signals may be applied to the input terminal 30.
In this case, if the over-large signal is applied to the gate electrode of the FET 21, a breakdown may be caused between the gate and source electrodes or the gate and drain electrodes of the FET 21 to result in the destruction of the FET 21. In order to prevent such destruction, the PIN diode 26 has conventionally been connected in parallel to the input side of the amplifier so as to limit the amplitude of the applied signal by using the saturation of the PIN diode 26. In addition, since the amplitude limited by the PIN diode 26 can be controlled by applying direct-current bias to the terminal 28 of the diode, such a PIN diode has often been employed for protecting the amplifier against over-large signals or keeping the linearity of the amplifier, in particular in power amplifier circuits.
Such FET amplifiers having a protector in the prior art bring about some problems. That is, it is necessary for the PIN diode 26 to be saturated in order to limit the amplitude of input signals, but the saturation characteristic thereof is not sharp enough, and therefore the limiting action is not sufficient, in particular when input signals vary over a wide range such as to go beyond the permissible limit of the FET 21 because of the gradual increase in the amplitude of the signals passing through the FET 21. Accordingly, it is essential for the saturation level of the PIN diode 26 to leave a substantial margin for any expectedly high level of the input signals. As a result, the capacity of the linearity of the amplifiers is not fully exercised and the range of linear operation is made narrow. Further, since the PIN diode 26 varies the phase of signals passing therethrough when it is saturated, a problem is experienced by the fact that the phase distortion increases as the amplitude of the input signals is changed, this problem being serious when the amplifier is used in a communication system.
Furthermore, the change in impedance of the PIN diode 26 caused by the saturation thereof varies the input matching condition and affects the frequency characteristics and the input-output behavioral characteristics.